Weed have been existing on the earth ever since the man started domesticating/ Cultivating plants and animals around 10,000 B.C.- The most significant single occurrence in human history. This was the beginning of farming and the farmers began to replace hunters about that time, a process continues till today.
Weed are the most underestimated crop pest in tropical agriculture although they cause higher reduction or loss in the yields of crops than other pests and diseases of the total annual loss of agriculture produces from various pest in India. Weeds roughly account for 45%.
Economic Importance of Weeds:-
Most definitions of weeds, for example, “all weeds, as such, are pernicious”. “Weeds are the plants which do more harm than good” Or ” Weeds as a class, have much in common with criminals, highlight and emphasize only the harmful aspect/ effect of weeds, although some weeds have beneficial role too. The economic important, therefore includes both harmful and beneficial aspects of weeds.
1.Harmful aspects of weeds:-
a) Reduction in crop yield:-
•Through competition with crops:
Weeds reduce the quantity of crop yield indirectly by competing with crops for light, spaces, nutrients and water.
•Through lodging of crops (mainly cereals):
Weedy crop is generally more vulnerable or prone than a non-weedy crop. Although lodging depends on the level of infestation growth habits and height of weeds in relation to crops. For example, this is of common occurrence if the growth habit and stature / height of weeds are similar to that of crops and infestation is medium to high.
It has been encountered that Avena ludoviciana , Phalaris minor induced lodging in wheat and Echinochloa sp and wild rice or red rice in rice.
b) Reduction in the quality of crop produces:-
Contamination / presence of weed seeds in crop seeds reduce or impair their quality and market value.
Ex:- Rotteboellia cochinchinesis in maize and rice.
c) Reduction in the quality of animal products:-
Weeds reduce the quality of animal products such as wool, milk, meat. Achyranthes aspera , Xanthium strumarium entangle with wool of sheep that graze on pastures.
d) Harbouring insect pest pathogens(diseases) and nematodes:
e) Increased cost of crop production :
f) Reduction in the economic value of land and water bodies:
g) Reduction in human efficiency:
Last but not least
h) Hazards to Aquatic animals:-
Aquatic animals particularly fish are highly affected due to huge algal growth or by other aquatic weeds, namely Eichnornia crassipes having profuse growth, which leads to suffocation or asphyxiation in those animals.
2.Beneficial Aspects of Weeds(Ethnobotany):-
Weeds do not possess are that bad or harmful. They have bright side too. Studying their beneficial aspects towards utilisation in a number of ways for humans and the society as a whole is called Ethnobotany.
•Weeds could be utilised as vegetables, potherb, fodder, fibre, oil, fuel, dyes, tannins and medicine or as a source of plant nutrients.
•Farmers have categorised 21 plants in cornfield as “mal monte” (Badweeds) and 20 as ” buen monte”( good weeds)
•The good weeds serve as food, medicines, ceremonial materials and or soil improves. Several useful and beneficial aspects of weeds as highlighted here.
a) Food for Human:-
Higher nutrients content of several weed species namely, (Chenopodium), (Amaranthus) have tempted humans since time immemorial to use them as a vegetables for fulfillment of the nutritional requirement. They are used as foods either partly or wholey by humans.
Ex:- (Amaranthus viridis), (Chenopodium album) are used as food/ vegetable in India.
b) Feed and Fodder for Animals:-
Weeds are used as fodder and feeds and important sources of nutrients to animals.
Ex:- (Trianthema portulacastrum)
c) Prevention of Soil Erosion:-
Weed control soil erosion by water and wind.
Ex:- (Cynodon dactylon), (Panicum repens) are excellent soil-binder weeds by virtue of the soil binding ability of their roots.
d) Green manuring, Composting and Mulching:
Several leguminous and non-leguminous weeds are used for green manuring and composting.
Ex:- (Ipomoea reptans), ( Eichnornia crassipes).
•The (Obnoxious parthenium) hysterophorus has also been found to have a good potential for green manuring. The recycling of weed biomass can be done directly as green manure or as compost. When incorporated into the soil at 5t/ha as green manure , it supplied 25 to 30kg N, 8-10 kg P and 45-50kg K.
” The weed was ready for direct incorporation with quick release of nutrients to crops”.
e) Nitrogen fixation:-
Some weeds under the Fabaceae fix atmospheric nitrogen into soil.
Ex:- (Melilotus indica), (Trigonella polycerate).
f) Nutrient recycling:-
Weeds are collected from the field on a large scale feed to the cattle. Their dung is collected and used as a source of plant nutrients. Roots of weeds mine/tap nutrients from the deeper layers of soil and returns them to soil surface as litter when they shed their leaves or the entire plant dies, decays or wither away. Then both roots and above ground shoots of weeds serve as sources of organic matter to soil.
g) Indicators of Nutrient deficiency :-
Several weed species are treated as markers of deficiency of different nutrients in the crop fields. While (chrysanthemum) and (Ranunculus), showing brown leaf margin are said to have K deficiency. (Chenopodium album) when grown with dull, dark, green leaves is treated as P- deficiency.
h) Medicinal value:-
Weeds are unwanted plants in agriculture, however, they are desirable as far as their medicinal value is concerned. Weed have great potential for use as medicine, but very little work has been carried out on this aspect.
Some weeds were indispensable in ancient medical herbal pharmacies and are still used for medicine today.
Ex:- (Strophanthus granatus) is a source of drug strophanthin (used for heart disease), (Cannabalis jalopa) extract is used for glaucoma andHeucas aspera) is used against snakebite.
Some more beneficial aspects of weeds are:-
•Ornamental values – (Gloriosa superb), (Bryophyllum sp).
•Pollution control – (Eichnornia crassipes)
•Sources of pesticides – (Chrysanthemum cinerarifolium)
•Meterials for cottage- is a source of pyrethrum
– (Cyperus pangores) – make mats
– (Typha angustata) – screens and mats.
• Source of Fuel, Fiber and Dye:-
_ (Jatropha gossypifolia) is the source of bio- diesel for automobiles.
•Traps for insect pest and pathogen:-
– It is known that weeds harbours or shelter insect pest and pathogen.
•Weeda helping pollination of crops:-
– weeds attract many beneficial insect such as honey bee, moths and serve as their hosts. These beneficial insects serve as pollination of our important crop.